1. Field of the Invention
The present invention relates to combustion exhaust filter traps, such as those f or Diesel particulate matter. More particularly the present invention relates to to an improved system and method therefor for removing particulate matter (PM) and unburned hydrocarbons (UHC) from the exhaust of combustion boxes, such as a municipal garbage incinerator. Still more particularly, the present invention relates to a combustion box exhaust filtration system and method which utilizes dual PM/UHC traps, low temperature regeneration ignition of the PM and associated UHC to burn the PM and UHC into CO and CO.sub.2 gases and recirculation of the burned PM and UHC back into the combustion box air intake for subsequent burning of any remaining UHC in the combustion box.
2. Description of the Prior Art
Increasingly garbage disposal has become an increasingly more difficult as land fills become ever more costly and unavailable. Accordingly, municipalities have turned to incineration to get rid of burnable garbage and produce electricity at the same time. Attendantly, environmental awareness has increasingly focused on limiting or eliminating offensive stack emissions from incinerators and other types of facilities and devices in which combustion occurs, hereinafter referred to inclusively as "combustion boxes". It is a well recognized problem that combustion produces a particulate matter soot, hereinafter referred to as "PM". PM is offensive to the average person, but actually, PM is composed mainly of carbon which, though not harmless, is less hazardous than unseen unburned hydrocarbons, hereinafter ref erred to as "UHC", which are known to be carcinogens. Thus, f or both the sake of the environment and the health of the public it is very desirable to reduce or eliminate both PM and UHC from the exhaust of combustion boxes.
An area of technology in which combustion box exhaust filtration has been studied extensively pertains to exhaust of Diesel engines. In the devices proposed in the prior art, there is provided a PM trap consisting in one form or another of a perforated metallic tube covered by a ceramic fiber filter. The metallic tube provides mechanical strength, while the ceramic fiber filter performs the actual PM removal. As engine operation proceeds, the PM trap accumulates progressively more PM. Problematically, the PM eventually clogs the PM trap resulting in loss of engine performance and, if left to continue unabated, would cause the engine to stall. Accordingly, there is provided some means for periodically removing the accumulated PM from the PM trap. This process of PM removal is referred to as "regeneration". Regeneration is typically accomplished by burning the PM at or above its ignition temperature of around 600 degrees Centigrade (in the presence of oxygen), which converts the PM into CO and CO.sub.2. During regeneration, the Diesel exhaust may dump to the atmosphere, or a second PM trap may be utilized in a cyclic fashion of operation. It will be appreciated in view of the foregoing, that a ceramic fiber filter is preferred as it can withstand the temperatures associated with regeneration, yet can trap PM which typically are on the order of 0.1 to 0.3 micrometers in size.
Two different approaches have been taken to accomplish periodic regeneration of PM traps. The first is to operate the PM Trap as close as possible to the PM ignition temperature, the second is to operate the PM trap at lower than the PM ignition temperature.
The theory behind operating a PM trap near the ignition temperature of PM is to permit rapid regeneration with very little additional energy being needed to provide ignition. The operating temperature of the PM trap is maintained high by locating it near the exhaust manifold, and either fuel or electricity is introduced to initiate ignition of the PM. In some systems, the Diesel engine RPM is increased so as to provide a suitably hot exhaust gas for ignition of the PM. Operation of the PM trap near the PM ignition temperature, while providing suitable burning of the PM, results in a very rapid burn process. This frequently leads to PM trap failure due to thermal shock, shortened life, melt-down, or poor operation under certain driving modes.
The theory behind operating a PM trap below the ignition temperature of the PM is to provide regeneration which is less injurious to the PM trap. In order to achieve ignition of the PM at a temperature below 600 degrees Centigrade, a PM oxidation promoting catalyst is introduced into the fuel, added as an exhaust gas chemical agent upstream of the PM filter, or as a pretreat for the ceramic fiber filter of the PM trap. These catalysts are certain metallic compounds, most notably composed of lead, copper, manganese, or noble metals, such as platinum and palladium.
Specific examples of the prior art will now be given.
U.S. Pat. No. 4,576,617 to Renevot, dated Mar. 18, 1986, discloses a Diesel exhaust PM trap which utilizes a regeneration process in which a very flammable mixture, such as methyl alcohol, is introduced into the PM trap which, in combination with a glow plug, effects ignition of the PM. Different mixtures may be used depending on whether the filter of the PM trap is impregnated with a catalyst, such as either platinum or palladium, as the ignition temperature of the PM will be different in accordance therewith.
U.S. Pat. No. 4,631,076 to Kurihara et al, dated Dec. 23, 1986, discloses a PM trap which utilizes regeneration based upon ignition of the PM caused by selective introduction of catalytic solutions into the exhaust gas upstream of the PM trap. Examples of suitable metal catalytic compounds include Pd(NH.sub.4).sub.3 (OH).sub.2 and Cu(NH.sub.3).sub.4 (OH).sub.2.
U.S. Pat. No. 4,685,291 to Ha, dated Aug. 11, 1987, and U.S. Pat. No. 4,813,233 to Vergeer et al, dated Mar. 21, 1989, disclose a dual PM trap system in which periodic regeneration may be achieved at lower than 600 degrees centigrade, where a by-pass conduit allows selectively for heated or cool-ed exhaust gasses to enter the PM traps. Four different ways to achieve regeneration are disclosed, as follows. 1) Each of the PM traps are located remote from the engine, but each is selectively heated by the other's exhaust manifold. When not heated, UHC can accumulate on the pores of the trapped PM. To effect regeneration, heat from the other PM trap's exhaust manifold is used to induce ignition of the PM, where it is believed by the inventor that the UHC serves as a fuel to assist ignition of the PM at temperatures as low as 250 degrees Centigrade. Exhaust coolers may be used in place of remote placement of the PM traps. 2) For two-stroke Diesel engines, regeneration is induced by a synergism between the scavenging blower system and introduction of finely atomized fuel above the PM traps, with a diesel fuel additive being used, such as manganese in concentrations on the order of 100 mg/L of diesel fuel. 3) Regeneration is induced by introduction of finely atomized fuel combined with air above the PM traps, with a diesel fuel additive being used, such as manganese in concentrations on the order of 80 to 100 mg/L of diesel fuel, or copper. 4) Again, for two-stroke Diesel engines, regeneration is induced by a scavenging blower system which controls the scavenging ratio of the engine, and introduction of finely atomized fuel above the PM traps, regeneration occurring because of increased exhaust gas temperature at medium load speed conditions, a diesel fuel additive being used, such as manganese in concentrations on the order of 100 mg/L of diesel fuel.
U.S. Pat. No. 4,720,972 to Rao et al, dated Jan. 26, 1988, discloses a dual PM trap utilizing a heat exchanger to cool the exhaust gases to the range between 200 and 500 degrees Fahrenheit, which promotes condensation of UHC upon the PM at the PM trap. The PM trap uses a catalytically coated ceramic fiber or wire mesh, where the catalytic material may comprise SO.sub.2 active oxidation catalyst such as platinum, tungsten or paladium-platinum coated on a porous, cellular cordierite body. Electrical heating is used to initiate ignition, and a burn front will progressively move down the PM trap from the ignition location until regeneration concludes in 6 to 9 minutes.
U.S. Pat. 4,730,454 to Pischinger et al, dated Mar. 15, 1988, discloses a PM trap in which regeneration is effected by regulating the PM concentration which lies within the explosive range of the PM/exhaust mixture by briefly adding or recycling combustible particulates to the exhaust gas flow at the PM trap. A secondary source of energy, such as electrical, is used to supply ignition, from which an explosive wave runs progressively through the PM trap.
While the schemes for cleaning Diesel exhaust are effective to remove PM, there is presently no successful system which can effectively remove both PM and UHC from Diesel exhaust.